Copper foil with low profile bond enhancement

ABSTRACT

A composite material, comprising a carrier strip the carrier strip comprising a first side the first side comprising a substantially uniform roughness, an electrolytically deposited copper foil layer having opposing first and second sides and a thickness of from 0.1 micron to 15 microns and the entire metal foil layer thickness having been deposited from a copper containing alkaline electrolyte, and a release layer effective to facilitate separation of the metal foil layer from the carrier strip disposed between and contacting both the first side of the carrier strip and the second side of the metal foil layer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 09/784,547 entitled “Copper Foil With Low ProfileBond Enhancement” by Szuchain Chen, et al. that was filed Feb. 15, 2001.This patent application relates to U.S. patent application Ser. No.09/522,544 entitled “Copper Foil Composite Including a Release Layer” bySzuchain Chen, et al. that was filed on Mar.10, 2000. The disclosures ofboth U.S. patent application Ser. Nos. 09/784,547 and 09/522,544 areincorporated by reference in their entireties herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a composite material having anintervening release layer. More particularly, a copper foil layer isreleasably bonded to a carrier layer for transport and assembly. Therelease layer disposed between the carrier layer and the copper foillayer facilitates separation. The copper foil layer may be laminated toa dielectric substrate in the manufacture of printed circuit boards. Alow height profile bond enhancing layer is formed on a surface of thecopper foil layer opposite the release layer.

[0004] 2. Description of Related Art

[0005] As electronic devices evolve, there is a need for thinner andsmaller printed circuits. This decrease in size leads to a requirementfor finer line spacing to increase circuit trace density.

[0006] Most printed circuit boards have a dielectric substrate, such asan epoxy or polyimide, laminated to a layer of copper foil. The copperfoil is etched into a desired circuit pattern. As the need for finerline resolution increases, thinner copper foil is required. This isbecause when copper foil is etched, etching occurs in both a verticaldirection and in a horizontal direction at about the same rate. Whilethe vertical etching is required to create spaces between adjacentcircuit traces for electrical isolation, horizontal etching at the sidesof a trace damages the integrity of the circuit traces. Horizontaletching limits the minimum line-to-line spacing to approximately twicethe thickness of the copper foil. Another problem with thicker copperfoil is that a longer time is required to etch the foil increasing themanufacturing cost and increasing the environmental concern due to thedisposal or reclamation of dissolved copper. Yet another problem arisesfrom the presence of Ni/Au overhang. Ni/Au overhang occurs when a Ni/Aucoating is applied as an etch resist and remains on the finished productto preserve solderability. When a thick copper foil coated with withNi/Au is etched, the etch undercut can be severe resulting in breakageof the Ni/Au overhang and possibly causing a short circuit in thefinished product.

[0007] One copper foil presently utilized in the manufacture of printedcircuit boards is referred to as one-half ounce foil. One square foot ofthis foil weighs approximately 0.5 ounce and has a nominal thickness ofabout 18 microns. Thinner copper foil, such as 9 micron thick foil, isavailable in the marketplace, however special care is required inhandling 9 micron foil to prevent wrinkling and damage.

[0008] Facilitating the handling of 9 micron, and thinner, foils is theuse of a carrier strip. The carrier strip is releasably bonded to thefoil for manufacturing and lamination. Once the foil is laminated andsupported by a dielectric, the carrier strip is removed. One commoncarrier strip is aluminum that may be removed by chemical etching, suchas by immersion in sodium hydroxide, without damage to the copper foil.Etching is time-consuming and disposal may create environmentalproblems.

[0009] Alternatively, a carrier layer, typically formed from copper, iscoated with a release layer. The copper foil layer is formed on therelease layer, typically by electrolytic deposition. Adhesion betweenthe release layer and the copper foil layer is high enough so that thecopper foil layer does not separate from the carrier layer prematurely,but is also sufficiently low that separation of the carrier layerfollowing lamination does not tear or otherwise damage the copper foillayer.

[0010] U.S. Pat. No. 3,998,601 to Yates et al. discloses a release layerformed from sulphides, chromates and oxides. An alternative releaselayer is disclosed to be chromium metal. U.S. Pat. No. 4,503,112 toKonicek discloses that chromium metal release layers have unpredictableadhesion and that preferred release layers include nickel, nickel/tinalloys, nickel/iron alloys, lead and tin/lead alloys. U.S. Pat. No.5,114,543 to Kajiwara et al. discloses a composite release layer havingan immersion deposited chromate layer that is coated with anelectrolytically deposited copper/nickel alloy. The U.S. Pat. Nos.3,998,601; 4,503,112 and 5,114,543 patents are incorporated by referenceherein in their entireties.

[0011] U.S. Pat. No. 5,066,366 to Lin discloses forming a release layeron a copper alloy foil carrier by treating the carrier with an aqueoussolution containing chromic acid and phosphoric acid. While a generallyacceptable process, areas of unacceptable high adhesion may occur when achrome phosphate release layer is formed directly on a copper alloycarrier. U.S. Pat. No. 5,066,366 is incorporated by reference in itsentirety herein.

[0012] There remains a need for an improved release layer thatconsistently provides adequate adhesion between a carrier layer and acopper foil layer to insure that the copper foil layer remains attachedto the carrier layer during transport and processing, such as laminationto a dielectric substrate. However, the adhesion to the release layer issufficiently low that the carrier layer may be removed followinglamination without damaging the copper foil layer.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is an object of the invention to provide a thinmetallic foil that is releasably attached to a carrier layer. A secondobject of the invention is to provide a method for the manufacture ofthe metallic foil/carrier layer composite. A further object of theinvention is to provide a thin copper foil useful for lamination to adielectric substrate for the manufacture of printed circuit boards andflexible circuits.

[0014] It is a feature of the invention that the metal foil isreleasably attached to a carrier layer and a force of at least 0.02pound per inch is required to separate the layers thereby insuring thatthe metal foil layer is not prematurely released. It is a furtherfeature of the invention that a maximum force of 2 pounds per inch, andtypically less than 1 pound per inch, is required to separate the metalfoil layer from the carrier layer thereby facilitating removal of thecarrier layer without damage to the copper foil layer.

[0015] A further feature of the invention is that the chemical solutionsutilized for deposition of the release layer are dilute aqueoussolutions that are believed to present less of an environmental hazardthan more concentrated electrolytes previously utilized to depositrelease layers such as metallic chromium.

[0016] Among the advantages of the invention are that the metal layermay be a thin copper foil with a thickness of 15 microns or less. Such athin foil facilitates the manufacture of printed circuit boards andflexible circuits with fine features. A further advantage is that thecarrier layer is mechanically separable from the metal foil layer anddoes not require etching for removal.

[0017] A further advantage is that the foils of the invention have lesssurface roughness than conventionally formed foils. As a result,undercutting during etching is reduced. Furthermore, the low-profiletreated surface is more suitable for high impedance/high frequencyapplications. The smoothness of the surface of the copper foil oppositethe release layer allows for improved imaging and fine line capabilityin the manufacturing of circuits.

[0018] In accordance with the invention, there is provided a compositematerial. The composite material has a support layer and a metal foillayer. A release layer is disposed between and contacts both the supportlayer and the metal foil layer. This release layer consists essentiallyof an admixture of a metal and a non-metal.

[0019] In one embodiment of the invention, the composite material isthen laminated directly to a dielectric substrate.

[0020] There is further provided a method for the manufacture of acomposite material that includes the steps of (1) providing anelectrically conductive support layer; (2) anodically treating theelectrically conductive support layer in a first aqueous electrolytethat contains first metal ions and hydroxide ions; (3) subsequentlycathodically depositing a release layer onto the electrically conductivesupport layer in a second aqueous electrolyte that contains second metalions and hydroxide ions; and (4) electrolytically depositing a metalfoil on the release layer.

[0021] One embodiment of this method of manufacture includes theadditional steps of laminating the metal foil layer to a dielectricsubstrate and then separating the electrically conductive support layerfrom the laminate at the release layer. The metal foil layer, now bondedto the dielectric layer, may then be formed into a plurality ofelectrically isolated circuit traces.

[0022] The above stated objects, features and advantages will becomemore apparent from the specification and drawings that follow.

IN THE DRAWINGS

[0023]FIG. 1 illustrates in cross-sectional representation a compositematerial in accordance with the invention.

[0024]FIG. 2 illustrates in cross-sectional representation the compositematerial of the invention laminated to a rigid dielectric as a precursorto a printed circuit board.

[0025]FIG. 3 illustrates in cross-sectional representation the compositematerial of the invention laminated to a flexible dielectric as aprecursor to a flexible circuit.

[0026]FIG. 4 is a perspective view of the printed circuit boardprecursor subsequent to removal of the carrier layer.

[0027]FIG. 5 illustrates in top planar view circuitry formed from thestructure of FIG. 4.

[0028]FIG. 6 illustrates an alternative release layer in cross-sectionalrepresentation.

[0029]FIG. 7 is a photomicrograph at a magnification of 5,000×illustrating a very low surface profile bond strength enhancement inaccordance with the invention.

[0030]FIG. 8 is a cross-sectional view of the very low surface profilebond strength enhancement of FIG. 7 embedded in a dielectric substrate.

[0031]FIG. 9 is a photomicrograph at a magnification of 5,000×illustrating a typical low surface profile bond strength enhancement asknown from the prior art.

[0032]FIG. 10 is a cross-sectional view of the low surface profile bondstrength enhancement of FIG. 9 embedded in a dielectric substrate.

[0033]FIG. 11 is a photomicrograph at 450× illustrating the surfacetopography of a dielectric layer upon removal of a very low surfaceprofile bond strength enhancement in accordance with the invention.

[0034]FIG. 12 is a photomicrograph at 450× illustrating the surfacetopography of a dielectric layer upon removal of a low surface profilebond strength enhancement as known from the prior art.

[0035]FIG. 13 is a photomicrograph at 450× of the surface topography ofa dielectric layer following removal of a low surface profile bondstrength enhancement illustrating incomplete copper removal.

[0036]FIG. 14 graphically illustrates release layer separation force asa function of lamination temperature.

DETAILED DESCRIPTION

[0037]FIG. 1 illustrates in cross sectional representation a compositematerial 10 in accordance with the invention. The composite material 10includes a support layer 12 and a metal foil layer 14. The support layer12 may be formed from any material capable of supporting the metal layer14. Preferably, the support layer 12 is formed from an electricallyconductive metal and has a thickness of at least 18 microns (1micron=1×10⁻⁶ meter). Suitable materials for the support layer includestainless steel, aluminum, aluminum alloys, nickel, copper and copperalloys.

[0038] When the support layer is stainless steel, a release layer, asdescribed below, may be optional.

[0039] Preferred for the support layer are copper alloys such as thosealloys designated by the CDA (Copper Development Association, New York,N.Y.) as copper alloy C110 (nominal composition by weight 99.95% copper(minimum) and 0.04% oxygen), copper alloy C715 (nominal composition byweight of 30% nickel and 70% copper), copper alloy C510 (nominalcomposition by weight of 94.5% copper, 5% tin and 0.2% phosphorous) andcopper alloy C102 (oxygen-free high copper having a minimum coppercontent of, by weight, 99.9%) as well as brasses, mixtures of copper andzinc containing up to 40%, by weight, of zinc.

[0040] Most preferably, the support layer 12 is a wrought material asopposed to electrolytically formed. The wrought materials tend to have ahigher strength and a higher stiffness facilitating handling of thematerials and enhancing peelability of a deposited foil. The supportlayer may be coated with a copper or nickel flash to cover up defects,such as incurred during rolling, that may interfere with the depositionor removal of the foil layer.

[0041] The support layer 12 may be a single material or a compositematerial with the second layer applied by any known process includingrolling, plating and sputtering. Combinations of copper and nickel orcopper and aluminum are believed useful as composite support layers.

[0042] Preferably, the thickness of the support layer 12 is from 18microns to 140 microns and more preferably from 35 microns to 70microns.

[0043] The metal foil layer 14 is any electrolytically deposited metalor metal alloy and is preferably copper. The metal foil layer typicallyhas a thickness of under 15 microns and more preferably is under 10microns. In accordance with one aspect of the invention, mostpreferably, the metal foil layer is electrolytically deposited from analkaline copper containing electrolyte followed by electrolyticdeposition from an acidic copper containing electrolyte and has athickness of from about 1 to about 6 microns and nominally about 5microns. In accordance with another aspect of the invention, the metalfoil layer is electrolytically deposited only from an alkaline coppercontaining electrolyte and has a thickness of from about 0.1 micron to 6microns and nominally about 5 microns. As described below, the metalfoil layer 14 may be deposited from a single electrolyte or fromcombinations of multiple electrolytes.

[0044] Disposed between and contacting both the support layer 12 and themetal foil layer 14 is a release layer 16. The release layer 16 consistsessentially of an admixture of a metal and a non-metal, with the bulkbelieved to be the non-metal. It is believed that the metal component ofthe release layer constitutes from 5% to 40%, by atomic percent.

[0045] Suitable metals are those that are reversibly, electrochemically,oxidizable in a suitable electrolyte, as opposed to dissolving. The listof suitable metals includes nickel, chromium, titanium, copper,manganese, iron, cobalt, tungsten, molybdenum and tantalum.

[0046] Preferred metals are nickel, chromium and mixtures thereof.Preferred non-metals are oxides, hydroxides, phosphates and chromates ofthe metals. Preferred combinations are mixtures of chromium and chromiumoxide, nickel and nickel oxide, chromium and chromium phosphate, nickeland nickel chromate, and nickel and nickel phosphate. The release layeris quite thin, on the order of 0.012 micron (120 angstroms) thick andtypically from about 0.001 micron to about 0.05 micron thick.

[0047] A most preferred release layer consists essentially of chromiumand at least one non-metal selected from the group consisting of oxidesof chromium and hydroxides of chromium. It has been established that theforce required to separate the metal foil from the carrier strip withthis most preferred release layer is consistently less than 7.1 kg/m(0.4 pounds per inch) even after exposure to temperatures of up to 380°C. for one hour. Since the metal foil is typically laminated, using heatand pressure, to a dielectric substrate prior to separation from thesupport layer, the release force as a function of temperature is animportant consideration.

[0048] Alternatively, the release layer 16 is a composite as illustratedin cross sectional representation in FIG. 6. A first portion 30 of therelease layer 16 is a metallic layer, as described above, and ispreferably selected to be nickel, chromium or a mixture thereof. Thisfirst portion 30 directly contacts the support layer 12 and is typicallydeposited by electroplating. Other methods of deposition such asimmersion plating or vapor deposition may also be utilized.

[0049] A second portion 32 of the release layer 16 is an admixture of ametal and a non-metal as described above. The second portion 32 directlycontacts the metal foil layer 14.

[0050] With reference back to FIG. 1, on a side 18 of metal foil layer14 opposite the release layer 16, a bond strength enhancing agent 20 maybe deposited. Suitable bond strength enhancing agents includeelectrolytically deposited copper dendrites or copper-nickel dendriteshaving a height of between about 0.5 and 2.7 microns and a height todiameter aspect ratio of between about 1 and 5. Such dendrites may beelectrolytically deposited from an aqueous solution containing copperions utilizing copper or lead electrodes with the composite material 10as the cathode. Pulses of DC current are applied between the anode andthe cathode as more fully described in U.S. Pat. No. 4,515,671 to Polan,et al., that is incorporated by reference in its entirety herein Otherbond strength enhancing agents include an electrolytically depositedmixture of chromium and zinc as disclosed in U.S. Pat. No. 5,230,932 toLin, et al., a silane based coating as disclosed in U.S. Pat. No.5,071,520 to Lin, et al., copper oxides, mechanical abrasion ofsurfaces, alternating current etching and micro-etching.

[0051] Bond strength enhancing processes that give a very low surfaceprofile, as achieved with the pulsed DC current of U.S. Pat. No.4,515,671, are preferred. Preferably, the average surface roughness(R_(a)) is 0.40 micron or less. The average surface roughness is definedas the arithmetic average value of all absolute distances of theroughness profile from the center line within the measuring length. Theroughness profile is determined utilizing a profilometer with a diamondstylus (contact method).

[0052] Typical low profile surface enhancements have a nodule height ofgreater than 3 microns and a nominal R_(a) value in excess of 0.4micron. The very low profile surface enhancement of the invention hassurface enhancements with a nodule height of between 0.5 micron and 2.7microns. Preferably, the maximum nodule height is between 1.8 micronsand 2.5 microns. The R_(a) value is less than 0.4 micron and preferablyR_(a) is between 0.20 and 0.35 micron.

[0053] The desirability of a lower surface profile is contrary to thetypical surface profile applied to relatively thick, on the order of 12micron, foil. Typically, higher surface profiles are utilized tomaximize peel strength once the copper foil is laminated to a dielectriccircuit board.

[0054]FIG. 7 is a photomicrograph at 5,000 times magnificationillustrating the very low surface profile bond strength enhancement ofthe invention. FIG. 8 is a cross-sectional view at a magnification of450 times that shows the bond strength enhancement 20 embedded intodielectric substrate 22.

[0055]FIG. 9 is a photomicrograph at 5,000 times magnificationillustrating a low surface profile bond strength enhancing agent asknown from the prior art. FIG. 10 is a cross-sectional view at amagnification of 450 times that shows the bond strength enhancing agent20 embedded into dielectric substrate 22.

[0056] As shown in FIGS. 11 and 12, when circuit traces 26 are formed byetching the metal foil layer, the exposed surface of dielectricsubstrate 22 has a topography that replicates the texture of the bondsurface enhancing agent. FIG. 11 is a photomicrograph of the surfacetexture resulting from etching a copper foil with the very low surfaceprofile bond enhancing agent of the invention. The magnification is 450times.

[0057]FIG. 12 is a photomicrograph at a magnification of 450 timesillustrating the surface of the dielectric substrate 22 followingremoval by etching of a copper foil with a typical low surface profilebond enhancement agent as known from the related art. A disadvantage ofthe typical low surface profile bond strength enhancing agent is moreapparent with reference to FIG. 13 that is also at a magnification of450 times. FIG. 13 illustrates that with the typical low surface profilebond strength enhancing agent, metallic dendrites may be embedded deeplyinto the dielectric substrate 22 and etching does not effectively removeall of the copper between adjacent circuit traces 26 leading to a shortcircuit 34.

[0058] Further, the smoother etch-exposed dielectric surface mirroringthe very low profile bond surface enhancement (FIG. 11) is much lesslikely to entrap contaminants and is much easier to clean as compared tothe more textured etch-exposed dielectric surface mirroring the typicallow profile bond surface enhancement (FIG. 12). Therefore, it should bepossible to achieve a higher surface insulation resistance (SIR)capability. A high SIR is important in the design of high densityinterconnect circuits where the circuit traces are routed closetogether, for example, 25-125 micron circuit traces separated by 25-100micron spaces.

[0059] For the manufacture of a printed circuit board, the compositematerial 10 is bonded to a dielectric substrate 22 forming circuitprecursor 36 as illustrated in FIG. 2. Metal foil layer 14 may belaminated through the addition of heat and pressure to a rigiddielectric for the manufacture of a printed circuit board. Typicallamination parameters are a temperature of about 180° C. for 50 minutes.Optionally, a polymer adhesive may assist in formation of the bond.Typical rigid materials for the dielectric substrate include fiberglassreinforced epoxies, such as FR4. The dielectric substrate may also be anelectrically conductive material coated with a dielectric material suchas a metal cored printed circuit board substrate or anodized aluminum.

[0060] Alternatively, as illustrated in FIG. 3, the dielectric substrate22 is a flexible polymer film such as a polyimide or polyamide. In thisinstance, the use of a polymer bond agent 24 such as acrylic or epoxypolymer is preferred. As in the preceding embodiment, metal foil layer14 is bonded to the dielectric substrate 22 forming circuit precursor36. Rather than laminating the flexible polymer to the metal foil layer,the flexible polymer may be cast on to the metal foil layer as a liquidor gel and cured to a flexible film.

[0061] After the composite material 10 is bonded to the dielectricsubstrate 22, the carrier layer 12 and release layer 16 are removed bymechanical means. Typically, removal is by applying a force to thecarrier layer/release layer in one direction and an opposing force tothe dielectric substrate/metal foil layer in a 90° direction. The forcesmay be either manually or mechanically applied. The force required forseparation, referred to as release force, is at least 0.02 pound perinch and preferably at least 0.05 pound per inch. This minimum releaseforce is required to prevent the metal foil layer 14 from separatingfrom the support layer 12 prematurely, such as during transport orduring bonding to the dielectric substrate. The release force shouldalso be less than 2 pounds per inch and preferably less than 1 pound perinch to ensure that during removal the metal foil layer remains adheredto the dielectric substrate 22 and does not tear or partially remainwith composite material 10. Preferably, the release force is between0.02 pound per inch and 2.0 pounds per inch and more preferably betweenabout 0.05 pound per inch and 1.0 pound per inch.

[0062]FIG. 4 is perspective view of a circuit precursor 36 with metalfoil layer 14 bonded to dielectric substrate 22. While FIG. 4 shows asingle metal foil layer bonded to the dielectric substrate 22,additional metal foil layers may be bonded to top surface 25 of themetal foil layer to form a multi-layer printed circuit board.

[0063] With reference to FIG. 5, the metal foil layer 14 may bechemically etched to form a printed circuit panel 44 having a pluralityof conductive features such as circuit traces 26 and die pads 28.Electrical isolation between conductive features is provided bydielectric substrate 22. Electrically conductive features may be formedby any process known in the art such as photolithography.

[0064] The following methods are useful for producing the compositematerial described above. It is recognized that variants of each methodmay be utilized and that different aspects of the various methods may bemixed together to produce a desired result. All methods generallyrequire appropriate degreasing or cleaning as a first step and rinsing,such as with deionized water, between appropriate steps.

[0065] In accordance with a first embodiment, a carrier strip formedfrom copper or a copper alloy has a thickness effective to support ametal foil layer. An exemplary nominal thickness for the carrier stripis approximately 35-70 microns. The carrier strip is immersed in adilute aqueous, alkaline sodium dichromate solution having theparameters specified in Table 1. All solutions disclosed herein areaqueous, unless otherwise specified. When a single value is given, thatvalue is nominal. TABLE 1 Sodium hydroxide 10-50 grams per liter (g/l)(broad range) 20-35 g/l (preferred range) Chromium ions, such as from0.1-10 g/l (broad range) sodium dichromate 0.5-5 g/l (preferred range)Operating Temperature 35° C.-50° C. PH Greater than 11 Counter ElectrodeStainless steel Voltage 1 volt-6 volts Current Density 0.5-10 amps persquare foot Anodic step (ASF) (broad range) 1-5 ASF (preferred range)Current Density 0.5-40 amps per square foot Cathodic step (ASF) (broadrange) 1-20 ASF (preferred range) Time (anodic step) 1-60 seconds (broadrange) 5-20 seconds (preferred) Time (cathodic step) 1-60 seconds (broadrange) 5-20 seconds (preferred)

[0066] The carrier strip is immersed into an electrolytic cellcontaining the electrolyte and a voltage is impressed across the cellwith the carrier strip as the anode. The anodic treatment generates auniform microroughness on the surface of the carrier strip and induces asubsequent uniform metal foil layer copper deposit. On completion of theanodic treatment, the carrier strip is maintained in the sameelectrolyte and the polarity of the electrolytic cell is reversed. Thecarrier strip is made the cathode to deposit a thin, on the order of10-500 angstrom, layer that is believed to be an admixture of chromiumand chromium oxides on the carrier strip. This admixture forms therelease layer that facilitates separation of the carrier stripsubsequent to lamination or other processing.

[0067] The release layer is formed to a maximum thickness of about 500angstroms. When the release layer thickness exceeds this maximum, theminimum release force requirements are not consistently achieved. Sincethe thickness of the release layer may be less than the microscopicsurface roughness of the copper foil, the precursor anodic treatment isused to achieve a more uniform surface finish.

[0068] Subsequent to rinsing, a seed layer of copper with a nominalthickness of between 0.1 and 0.5 micron of copper is deposited on therelease layer utilizing the parameters specified in Table 2. TABLE 2Copper ions, such as from 5-35 g/l (broad range) copper sulfate and/or15-25 g/l (preferred copper pyrophosphate range) Optional inclusions ofAmount as required leveling agents, complexing agents and surfactantsOperating Temperature 35° C.-70° C. pH 6-10 Anode Material StainlessSteel or copper voltage 3-7 volts Current Density 10-50 ASF Time 5-100seconds

[0069] The seed layer forms a nucleating agent for the subsequent highspeed deposition of a copper foil layer. While the seed layer ispreferably formed from copper, it may be any electrically conductivemetal that can be deposited in a mildly acidic to alkaline solution andis etchable in the same chemical solutions as copper. Such other metalsfor the seed layer include copper alloys, tin, iron, zinc, nickel,cobalt, etc. The seed layer protects the release layer from chemicalattack in an electrolyte utilized to deposit the bulk of the metal foillayer thickness. Typically, to maximize manufacturing speed, an acidcopper electrolyte as specified in Table 3 is utilized.

[0070] Alternatively, immersion time in the copper containing alkalineelectrolyte, such as that disclosed in Table 2, is increased to on theorder of 100 seconds to 20 minutes to deposit a copper layer with athickness of from approximately 1.0 to 15 microns. In this aspect of theinvention, the subsequent step of building up the copper thickness in acopper containing acidic electrolyte is omitted. TABLE 3 Copper ions,such as from 20-80 g/l (broad) copper sulfate 50-70 g/l (preferred)Sulfuric acid 30-200 g/l (broad) 40-100 g/l (preferred) OperatingTemperature 25° C.-70° C. PH Less than 1.5 Anode Material Lead or copperVoltage 5-10 volts Current Density 30-1000 ASF (broad) 40-500 ASF(preferred) Time 0.5-8 minutes (broad) 1-5 minutes (preferred)

[0071] To enhance adhesion, a dendritic treatment may be used to roughenthe outside surface of the metal foil layer. One suitable dendritictreatment utilizes the parameters specified in Table 4. Alternatively,an anti-tarnish layer such as a mixture of chromium and zinc may bedeposited to increase adhesion without increasing surface roughness.TABLE 4 Copper ions, such as from 15-70 g/l (broad) copper sulfate 18-25g/l (preferred) Sulfuric acid 10-200 g/l (broad) 35-100 g/l (preferred)Sodium lauryl sulfate 1-20 ppm Operating Temperature 25° C.-55° C. pHLess than 1.5 Anode Material Lead or copper Voltage 5-10 volts CurrentDensity 50-1000 ASF (broad) 100-500 ASF (preferred) Time 4-60 seconds(broad) 4-40 seconds (preferred)

[0072] In a second embodiment, a carrier strip as described above isimmersed in the solution of Table 1 for a time of from two to sixtyseconds without utilizing electric current. A nominal 5 micron copperfoil layer and dendritic treatment is then applied as above.

[0073] In accordance with a third embodiment of the invention, a coppercarrier strip, as described above, is electrolytically coated with athin, on the order of between 0.05 micron and 2 microns, layer of nickelutilizing the parameters described in Table 5. TABLE 5 Nickel sulfamate150-600 g/l (broad) 400-500 g/l (preferred) Nickel chloride 0-15 g/l(broad) 0

7 g/l (preferred) Boric acid 25-50 g/l (broad) 35-45 g/l (preferred)Operating Temperature 45° C.-60° C. pH 2-5 Anode Material Nickel orStainless Steel Voltage 0.5

5 volts Current Density 20-60 ASF Time 20-60 seconds

[0074] A chromium phosphate release layer is then applied over the thinlayer of nickel by immersion in a dilute chromic acid/phosphoric acidsolution having the parameters disclosed in Table 6. TABLE 6 Chromicacid 0.1-20 g/l (broad) 0.2-10 g/l (preferred) Phosphoric acid 0.1-80g/l (broad) 0.5-40 g/l (preferred) Operating Temperature 20° C.-60° C.pH 0.1-3 Time 5-120 seconds (broad) 10-40 seconds (preferred)

[0075] A nominal 5 micron copper foil metal layer is then deposited asabove followed by a dendritic treatment as above.

[0076] In accordance with a fourth embodiment of the invention, a thin,on the order of between 0.05 micron and 2 microns, layer of nickel isdeposited on a copper alloy carrier strip as above. A release layer isdeposited from an aqueous solution containing sodium hydroxide asdisclosed in Table 7. TABLE 7 Sodium hydroxide 10-80 g/l (broad) 20-50g/l (preferred) Operating Temperature 35° C.-60° C. pH Greater than 11Counter Electrode Stainless steel Voltage 0.5-5 volts Current Density(anodic 1.0-50 ASF (broad) step) 2.5-35 ASF (preferred) Current Density(cathodic 0.5-40 ASF (broad) step) 1.0-25 ASF (preferred) Time (anodicstep) 2-60 seconds 5-30 seconds Time (cathodic step) 2-60 seconds 5-30seconds

[0077] The nickel coated carrier strip is first made anodic and thencathodic to form reduced nickel oxides. Approximately 5 microns ofcopper is then applied as the metal foil layer followed by a dendritictreatment as described above.

[0078] In each of embodiments 1-4, an alkaline copper plating bath waspreferably used to deposit a seed layer having a thickness of from about0.1 to about 0.5 micron of copper prior to depositing up to 5 microns ofcopper plating in an acidic bath. In the alternative first embodiment,the subsequent deposition of copper from an acidic bath is omitted. Theinitial use of an alkaline copper bath avoids potential attack to thechromium oxide, nickel oxide or nickel phosphate release layer as couldhappen in the acidic copper bath thus improving thereliability/integrity of the release interface. Embodiments five and sixdescribe methods for forming a composite material having similarreliability and integrity without the need for an alkaline copper bath.

[0079] In embodiment five, a smooth nickel deposit is formed on thecopper alloy carrier strip utilizing a suitable nickel plating bath,such as the nickel sulfamate electrolyte of Table 5. The nickel platedcarrier strip is then immersed in an aqueous electrolyte containingsodium hydroxide utilizing the parameters recited in Table 7. Thecarrier strip is first made anodic and then cathodic. A copper metalfoil layer is then deposited using a copper sulphate bath (Table 3)followed by dendritic treatment (Table 4).

[0080] In a sixth embodiment, a thin layer of nickel, having a thicknesson the order of between 0.05 micron and 2 microns, is applied to thecarrier strip (Table 5) as described above followed by treatment in anaqueous solution containing sodium hydroxide with the carrier stripfirst forming the anode and then the cathode as in Table 7. Next, thenickel is treated cathodically in an acid copper sulfate bath at lowcurrent density and the parameters illustrated in Table 8. TABLE 8Copper ions, such as from 40-80 g/l (broad) copper sulfate 60-70 g/l(preferred) Sulfuric acid 50-100 g/l (broad) 60-75 g/l (preferred)Operating Temperature 35° C.-60° C. pH Less than 1 Cathode Material Leador copper Voltage 5-8 volts Current Density 0.03-2 ASF (broad) 0.05

0.5 ASF (preferred) Time 30-120 seconds (broad) 45-90 seconds(preferred)

[0081] Copper deposition as in Table 3 is then utilized to increase thethickness up to a nominal 5 microns. Dendritic treatment as in Table 4completes the process.

[0082] Composite materials formed from any one of the above processesmay then be used to manufacture either printed circuit boards or flexcircuits as described above. The advantages of the invention will becomemore apparent from the examples that follow.

[0083] In an alternative embodiment of the invention, the presence ofpinholes in the foil may be reduced by employing methods of surfacepreparation on the exposed surface of the carrier strip that result in asmoother, more uniform surface. Examples of such methods of preparationinclude making use of oxygen free copper for the carrier strip, as wellas smooth rolling, micro-etching, and flashing the surface of thecarrier strip with copper or nickel solutions.

[0084] The presence of oxygen in copper carrier strips can give rise theformation of pits in the exposed surface of the carrier strip arisingfrom the presence of copper oxide particles. When the release layer andfoil are deposited over these pits, they conform to the structure of thepits. As a result, when the carrier strip is removed from the foil, theportions of the foil deposited into the pits often times break off fromthe foil. This breakage can result in pinholes on the surface of thefoil or undesirable imperfections to the otherwise relatively uniformsurface of the foil.

[0085] Therefore, one embodiment of the present invention incorporatesthe use of oxygen free copper for the carrier strip. The use of oxygenfree copper reduces or largely eliminates the presence of copper oxideparticles, thus reducing the attendant pits, and providing for a moreuniform foil surface both before and after separation from the carrierstrip.

[0086] Additional methods employed in the present invention to produce amore uniform carrier strip surface include smooth rolling, microetching,and exposing the surface of the carrier strip onto which the releaselayer is to be deposited to a copper or nickel flash. Examples ofmicroetching capable of smoothing the surface of the carrier strip to adesirable uniformity include, but are not limited to, the application ofa 1 lb./gal. ammonium persulfate solution with 3 v % sulfuric acidmixture at 115° F. for approximately 50 seconds. As stated above, thecarrier strip may be coated with a copper or nickel flash to cover updefects, such as those incurred during rolling, that may adverselyimpact the uniform roughness of the surface of the carrier strip.

[0087] In another embodiment of the present invention, a dark andnon-reflective layer is interposed between the release layer and thefoil. The dark and non-reflective coating remains bonded to the foilafter separation from the carrier strip and enhances the ability of thecopper foil to be fashioned through the use of a laser.

[0088] There are, in general, two types of lasers that are used to drillthrough or otherwise remove portions of metallic foil. The first typeincludes CO₂ lasers which emit light in the IR range. The second type oflasers includes YAK lasers which emit light in the UV range. While YAKlasers can drill directly through copper foil, such lasers tend to do soat a relatively slow rate. In contrast, CO₂ lasers are capable oftransmitting more energy per unit of time to the foil and, hence,drilling through the foil at a relatively faster rate. However, CO₂layers are generally incapable of drilling through copper foil directly.Rather, in order to drill through metallic foil, a CO₂ laser requiresthat the surface of the foil be treated in such a manner as to render itboth dark and non-reflective. As used herein,

non-reflective

refers to the property exhibited by a surface that substantiallyabsorbs, rather than reflects, the energy contained in light whichinteracts with the surface.

[0089] Therefore, with reference to FIG. 15, in one embodiment of thepresent invention, there is interposed between the release layer 16 andthe metal foil layer 14 a dark layer 13 comprised of dark material.After the carrier layer 12 and release layer 16 are removed, the darklayer 13 remains bonded to the metal foil layer 14 to facilitatedrilling by a CO₂ laser. The dark layer 13 is preferably formed ofbetween approximately 0.05 and 0.5 microns of a nickel-coppercombination, cobalt, tin, manganese, iron, or nickel layer. Mostpreferably the dark layer is approximately 0.2 microns in thickness. Inorder to render the dark layer 13 non-reflective, the surface of thecarrier strip 12 opposite the dark layer 13 is made uniformly rough.Because the surface texture of the carrier strip 12 is imprinted uponthe side of the dark layer opposite the attached foil, the dark layerassumes a surface texture similar to that of the uniformly rough carrierstrip 12 surface.

[0090] There exist several methods by which the surface of the carrierstrip 12 can be made suitably uniformly rough. These methods include,but are not limited to, nodule plating the surface of the carrier stripwith pink or black copper bonding such as CopperBond

(a registered trademark of the Olin Corporation of Norwalk, Conn.),sandblasting the surface of the carrier strip, microetching the surfaceof the carrier strip, and rough rolling the carrier strip.

[0091] The resultant non-reflective dark layer facilitates the use of alaser, preferably a CO₂ laser, to drill through the dark layer and theunderlying foil layer. In addition to drilling holes in this manner, thelaser may be manipulated to etch into the foil a desired circuit schemasuch as one required to form an integrated circuit.

EXAMPLES Example 1

[0092] A 2 oz. wrought copper foil was used as a carrier strip. Thestrip was electrocleaned in an alkaline commercial cleaner using 20 ASFcurrent density for 40 sec. The foil was rinsed and then the releaselayer treatment was conducted in 20-35 g/l NaOH+0.5-5 g/l chromium ionsas sodium dichromate solution using an anodic current of 1-5 ASFfollowed by a cathodic current of 1-20 ASF for 5-20 sec. The anodictreatment appeared to generate a uniform micro-roughness on the surfaceof the foil and induce a uniform copper deposit. The cathodic treatmentappeared to deposit a transparent layer of chromium and chromium oxides,which is believed to be responsible for the release of the carrier stripafter lamination.

[0093] A seed layer of 0.1-0.5 micron copper was electroplated in analkaline copper plating solution. A 5 micron copper deposit was thenelectroplated, using 60-70 g/l copper ions as copper sulfate and 60-75g/l sulfuric acid at 40-100 ASF for 5.4-2.1 minutes, followed by adendritic copper or copper/nickel treatment. After lamination to an FR-4epoxy substrate, the 2 oz carrier was easily peeled with a measured bondstrength of 0.1-1.0 lb/in.

Example 2

[0094] A 2 oz. wrought copper foil was used as a carrier strip. Thestrip was electrocleaned in an alkaline commercial cleaner using 20 ASFcurrent density for 40 sec. The foil was rinsed and then the releaselayer treatment applied by electroplating in a solution of 20-35 g/lNaOH+0.5-5 g/l chromium ions as sodium dichromate. This treatmentappeared to form a transparent layer of chromium and chromium oxides.

[0095] A seed layer of 0.1-0.5 micron copper was electroplated in analkaline copper plating solution. A 5 micron copper deposit was thenelectroplated using 60-70 g/l copper ions as copper sulfate and 60-75g/l sulfuric acid at 40-100 ASF for 5.4-2.1 minutes, followed by adendritic copper or copper/nickel treatment. After lamination to an FR-4epoxy substrate, the 2 oz. carrier was easily peeled with a measuredbond strength of 0.1-1.0 lb/in.

Example 3

[0096] After cleaning the copper carrier strip, a nickel layer was firstelectroplated with 0.15 micron nickel in a nickel sulfamate bath at 30ASF for 20 sec. The foil was then immersed in a solution containing0.2-10.0 g/l chromic acid and 0.5-40 g/l phosphoric acid for 10-40 secat ambient temperature. The alkaline copper seed layer and acidic copperplating were conducted as described in Example 1. A peelable foilresulted after lamination with 0.2-2.0 lb/in release force.

Example 4

[0097] As in Example 3, a nickel layer was first electroplated. Thenickel surface was then anodically treated in a 20-50 g/l NaOH solutionat 0.5-10 ASF for 5-30 sec to generate a nickel oxide release layer.This nickel oxide layer was then cathodically reduced in a 20-50 g/lNaOH solution at 0.5-50 ASF for 5-30 sec. This cathodic treatmentappeared to produce reduced oxides and enlarge the operating window.Without the cathodic treatment, if the anodic current is too low, anon-peelable foil would be produced. If the anodic current is too high,the plated 5 micron foil often delaminates or forms blister even beforelamination and renders the product useless.

[0098] After the nickel and nickel oxide treatment, the alkaline copperseed layer and 5 micron acidic copper are deposited. A release force of0.35-1.0 lb/in was obtained.

Example 5

[0099] The formation of a smooth nickel deposit onto a carrier strip hasbeen shown to be very easily accomplished by deposition out of a nickelsulfamate bath using a current density of 30 to 50 ASF with times of 20to 30 seconds.

[0100] Treatment of the nickel plate in a solution of 30 g/l sodiumhydroxide was then conducted with an anodic treatment of 20 to 40 ASFfor 20 to 40 seconds with subsequent cathodic treatment in the samesolution at 25% to 50% of the anodic current density and for half thetime. No seed layer was used, rather the nickel coated support layer wascathodically treated in acid-copper sulfate bath at low current density,60 seconds at 0.03 to 2 ASF, followed by plating of copper from the acidcopper sulfate bath at 65 ASF for 3.5 minutes to achieve the desired 5micron thickness. The foils retained their peelability followinglamination.

[0101] For comparison, it was demonstrated that the above low currenttreatment resulted in peelable foils free of defects while foils madeusing identical conditions, but without the low current treatment, werefull of defects and not peelable following lamination.

Example 6

[0102] The effect of the release layer on the separation force requiredto separate a copper foil layer from a copper support layer isgraphically illustrated in FIG. 14. Both a chromium metal release layer,as known from the prior art, and a chromium plus chromium oxide releaselayer as disclosed herein were effective to provide a relatively low,less than 8.9 kg/m (0.5 pounds per inch), separation force at roomtemperature. Even when heated to temperatures of up to 200° C. andremaining at temperature for one hour, the separation forces are aboutequal. However, at temperatures above 200° C., the separation force forthe chromium release layer (reference line 38) increases rapidly whilethere is no increase in the separation force for the chromium pluschromium oxide release layer (reference line 40).

[0103] This example illustrates the wider processing window availablefor lamination utilizing the release layer of the invention.

Example 7

[0104]FIGS. 8 and 10 illustrate that more vertical side walls 42 areobtained utilizing the 5 micron foil as compared to the 12 micron foil.To avoid the presence of short circuits, etching must be for a timeeffective to ensure substantial removal of copper from the surface ofthe dielectric 22. Since the surface of the foil opposite the dielectriccarrier 22 is exposed to the etching solution for a longer time than thesurface adjacent the dielectric carrier, a circuit trace width reductionat that opposing surface occurs. Due to the longer etching time for the12 micron foil, this over-etching is more pronounced.

[0105] Table 9 demonstrates conductive circuit traces having a nominalwidth as indicated when formed by photopatterning a 38 micron thick dryfilm photo resist and then etching in an alkaline solution at atemperature of 52° C. The 5 micron foil required approximately half theetch time of the 12 micron foil. From Table 9, it may be seen that theconductor width is closer to that of the designated conductor width for5 micron foil than for 12 micron foil. TABLE 9 Etched Conductor DesignedConductor Width Width (μm) (μm) 5 μm foil 12 μm foil 50 40 24 75 63 52100 92 76 125 117 101

[0106] TABLE 10 % Fewer Defects Feature (5 μm vs. 12 μm foil)  50 μmconductors 30  75 μm conductors 4 100 μm conductors 65 125 μm conductors62  50 μm spaces 31  75 μm spaces 19

[0107] TABLE 11 After After lamination lamination Sample Anodic/CathodicC7025 C110 ID Current (asf) carrier carrier 1 1.7 Cracked Weak openrelease 2 1.25 Weak Weak release release 3 1.1 Cracked Weak open release4 1.0 Weak No release release 5 0.8 Cracked No open release 6 0.6 Norelease No release

[0108] A 1 ounce C110 foil and 1 ounce C7025 foil were utilized to formcarrier strips. Prior to cleaning in NaOH the foil samples were immersedin 10% sulfuric acid to remove any surface oxides. To provideconsistency none of the solutions were stirred. As the objective of thetest was to compare the capability of foils of differing tensilestrengths in providing a controllable release force, the parametersunder which the tests were conducted were held constant while the twofoils differed only in tensile strength. The tensile strength of C7025was approximately 100 ksi after lamination and the tensile strength ofC110 was approximately 30 ksi after lamination.

[0109] As table 11 illustrates, the release force of both foils isdependent upon the anodic/cathodic current parameters. The break offpoint for anodic/cathodic current is 0.8 asf for the C7025 foil and 1.1for the C110 foil. As used herein, “break off point” refers to theminimum anodic/cathodic current treatment value at which release isenabled.

[0110] It is apparent that there has been provided in accordance withthe present invention a composite material including a releasable metalfoil layer and methods for the manufacture of such a composite materialthat fully satisfies the objects, means and advantages as set forthherein above. While the invention has been described in combination withembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alternatives, modifications and variations as fall within thespirit and broad scope of the appended claims.

We claim:
 1. A composite material, comprising: a carrier strip saidcarrier strip comprising a first side said first side possessing asubstantially uniform roughness; an electrolytically deposited copperfoil layer having opposing first and second sides and a thickness offrom 0.1 micron to 15 microns and said entire metal foil layer thicknesshaving been deposited from a copper containing alkaline electrolyte; anda release layer effective to facilitate separation of said metal foillayer from said carrier strip disposed between and contacting both saidfirst side of said carrier strip and said second side of said metal foillayer.
 2. The composite material of claim 1 wherein said substantiallyuniform roughness results from smooth rolling said first side of saidcarrier strip.
 3. The composite material of claim 1 wherein saidsubstantially uniform roughness results from microetching said firstside of said carrier strip.
 4. The composite material of claim 3 whereinsaid microetching of said first side of said carrier strip comprises theapplication of ammonium persulfate.
 5. The composite material of claim 3wherein said microetching of said first side of said carrier stripcomprises the application of a sulfuric acid mixture.
 6. The compositematerial of claim 1 wherein said substantially uniform roughness resultsfrom flashing said first side of said carrier strip with copper.
 7. Thecomposite material of claim 1 wherein said substantially uniformroughness results from flashing said first side of said carrier stripwith nickel.
 8. The composite material of claim 1 wherein said carrierstrip is comprised of oxygen free copper.
 9. The composite material ofclaim 1 wherein said carrier strip is comprised of acopper-nickel-silicon based alloy.
 10. The composite material of claim 1wherein said carrier strip has a tensile strength of at least 30 ksi.11. The composite material of claim 1 wherein said carrier strip has atensile strength of at least 100 ksi.
 12. A composite material,comprising: a carrier strip said carrier strip comprising a first sidesaid first side possessing a substantially uniform roughness; anelectrolytically deposited copper foil layer having opposing first andsecond sides and a thickness of from 0.1 micron to 15 microns and saidentire metal foil layer thickness having been deposited from a coppercontaining alkaline electrolyte; a dark layer effective to absorb lightenergy said dark layer having opposing first and second sides said firstside of said dark layer in contact with said second side of said copperfoil layer; and a release layer effective to facilitate separation ofsaid carrier strip from said dark layer disposed between and contactingboth said first side of said carrier strip and said second side of saiddark layer and effective to transmit the surface characteristics of saidfirst side of said carrier strip to said second side of said dark layer.13. The composite material of claim 12 wherein said substantiallyuniform roughness results from rough rolling said first side of saidcarrier strip.
 14. The composite material of claim 12 wherein saidsubstantially uniform roughness results from microetching said firstside of said carrier strip.
 15. The composite material of claim 12wherein said substantially uniform roughness results from sand blastingsaid first side of said carrier strip.
 16. The composite material ofclaim 12 wherein said substantially uniform roughness results fromnodule plating said first side of said carrier strip.
 17. The compositematerial of claim 12 wherein said dark layer has a thickness between0.05 and 0.5 microns.
 18. The composite material of claim 17 whereinsaid dark layer has an approximate thickness of 0.2 microns.
 19. Thecomposite material of claim 12 wherein said dark layer is comprised of amaterial selected from the group consisting of copper, nickel, tin,manganese, iron, and copper-nickel alloys.
 20. A method for themanufacture of a composite material comprising the steps of: providingan electrically conductive support layer; anodically treating saidelectrically conductive support layer in a first aqueous electrolytecontaining first metal ions and hydroxide ions; subsequent to saidanodically treating step, cathodically depositing a release layer ontosaid electrically conductive support layer in a second aqueouselectrolyte containing second metal ions and hydroxide ions; andelectrolytically depositing a metal foil layer on said release layer byimmersion in a copper containing alkaline electrolyte for a period oftime sufficient to achieve a thickness of said metal foil layer ofapproximately 1.0 to 15 microns.